2002 8.1 Gi-b overheats at WOT. Volvo Penta duo prop DPSM Outdrive. Engine rebuilt for cracked piston ring less than 100 hours ago, but about 12 years ago. Seawater cooling system. Boat stored in heated/cooled shop. Inland freshwater lakes only, no saltwater.

Symptoms:

When idle for extended periods temp gauge stays at 150 (140 degree thermostat). Gradually increase throttle to half throttle cruise on plane, stays at 150 for five minutes, then creeps up to 170. Increase to full throttle. Temp creeps up towards 200 within a couple minutes and alarms/derates motor.

It takes almost 10 minutes at idle for temp to drop back to 150.

Steps taken (in order from water intake to exhaust):

Replaced outdrive water pickup bushing, o-rings, cleanout cover gasket and rubber hose. No trash or dirt found. inspected and re-used plastic nipple.

Replaced raw water pump assembly with new OEM pump.

Disconnected intake hose from raw water pump and backflushed power steering cooler and outdrive - all clear.

Did NOT do clear hose intake test to check for air leaks, BUT pressurized system in reverse from raw water pump back to outdrive intake, no leaks.

Inspected transom hose for kinking/collapse.

Cleaned oil cooler intake side, removed a couple of small impellor pieces. Shop-vac'd a gallon of water through cooler in reverse direction.

Replaced thermostat (140 degree) and o-rings, cleaned housing (not terribly corroded).

Reverse flushed engine block drain (winterize) system for 10 minutes - no rust flake or grit.

Removed and inspected circulation pump. No corrosion or wear on vanes, tight on shaft. Replaced with new gaskets. Engine block water inlets clean (light rust color) and free of rust flakes.

Removed/inspected starboard exhaust riser, light rust color, no rust flakes. Power washed, no rust flakes or grit. Water flows freely through riser.

Top of exhaust manifolds are clear of any rust flake.

Replaced riser temp sensors.

Have NOT removed inspected bottom of exhaust manifolds water galleries.

Ran engine with props off, in tub. Suction from water inlet is obvious. Half throttle suction is great. Water output from exhaust and overflow is significant. Video available.

Compression test resulted in cylinder pressures 158-165. No wet compression test.

Leak down results 4-7%. (By this time, I was hoping for a head gasket issue.) Spark plugs are on the dirty/cool side, but uniform.

Since temp gauge and ECU have separate sensors, it seems like they are in sync when engine gets hot, then derates. Therefore, probably not either of those sensors.


HELP!

The hose from the outdrive to the seapump could possibly be collapsing under suction from the sea pump. The first thing I would do is hook up a mechanical water pressure gauge, either permanent, or at least temporary. Don't know a lot about what data you can see with a scanner hooked up on your engine, but you may be able to see water pressure as well.

Not sure if it happens with Volvo's but do a search for bravoitis. My friend had nearly the exact same thing happening as you and his transom hose was partially plugged.

Have you put and IR temp gun on the motor to confirm its actually over heating?

Remove side plates on heat exchanger on front/top of engine. Most likely the rubber gaskets are all swollen and blocking flow. Very common problem.

No water pressure sensor or gauge on this one. I can probably hook up a temporary one inline. I thought about mounting a GoPro back there to watch that hose. Wish they put a stainless steel spring inside or a stiffer hose. What did your friend do? Just replace the hose?

Thanks, but it’s seawater cooled, no exchanger.

I’ll take an IR gun on next test ride. Odd thing is that I can hold my hand on hoses and exhaust risers. I figured that since there are two temp sensors on thermostat housing, one for analog gauge and one for ECU, that they are working properly because gauge goes up then engine alarms/derates.

If you have the funds, pickup a Vesselview mobile dongle ($100 on ebay). It has a bluetooth app and will show you your ECU data in real time including water temp, water PSI, Oil PSI, fuel consumption etc. Nice to have if you cant swing a rinda tool

Which intake hose typically collapses, (13) ps cooler to transom or (6)raw pump to ps cooler? My luck it will be the bottom one that’s nearly impossible to get to.

OP indicates it is a Volvo Penta so VV is not gonna work.

I had the 8.1Gxi version of this motor and chased similar over and under heating problems with the seawater cooling. I converted to closed cooling (VP makes a kit) and it solved all cooling problems. 496's like closed cooling for the consistent temps.